1. Field of the Invention
This invention relates to glass manufacturing and more particularly to improved apparatus and methods of melting glass.
2. Description of the Prior Art
Ecological and energy source considerations have brought the electrical melting and refining of glass to a position of significance in glass technology. Stack gases from fossil fuel fired glass melters contain both the products of combustion and gases evolved from the melting and refining of the glass constituents. It is desirable to minimize such gases. One approach which reduces the gases evolved from glass melting and refining is to employ Joule effect heating of molten glass and to maintain a blanket of thermally insulating glass batch on the upper surface of the molten glass. This type of operation has been termed "cold top" melting and refining. The continuous melting of glass by "cold top" processes can be considered to comprise charging batch at the top of the mass of glass constituents, melting the batch at its interface with the underlying molten glass, and withdrawing the glass at the bottom of the glass mass through a submerged passage or throat. Batch dust from the batch charging operation and from the passage of gases from the molten glass through the batch blanket can be a problem in cold top operation. Frequently, further refining and/or conditioning of the glass is required after it is withdrawn from the melting tank or region to eliminate gas which produces bubbles or seeds in the products formed.
In more common prior glass melting processes where fossil fuel firing has been employed, melting of the glass batch occurs at the upper surface of the mass of glass constituents and the glass batch added to the mass is ordinarily confined to a restricted portion of that upper surface until it is melted into the body of molten glass. Such melting reduces the amount of dusting of batch and increases the opportunity for the evolution of seed producing gas from the molten mass so that refining of the glass can be more complete in the melter. Glass can be withdrawn at the upper surface of the melt through a surface throat without entraining intolerable quantities of unmelted batch or inadequately refined glass.
Glass furnaces employed in commercial continuous melting processes include equipment and structure which exceed the life of the refractory structure of the glass tank and therefore can be reused. Conditions have dictated modifications toward a cold top type of operation even in the case of furnaces originally constructed for fossil fuel firing. In U.S. Pat. application Ser. No. 452,900 filed Mar. 20, 1974, now U.S. Pat. No. 3,885,945, in the names of Vernon C. Rees and Magnus L. Froberg entitled "Method of and Apparatus for Electrically Heating Molten Glass" glass melting and refining apparatus and techniques are disclosed which combine features of cold top operation and fossil fuel melting applicable to a furnace framework and auxiliary equipment configuration originally intended for fossil fuel melting and refining. In that arrangement the preponderant heat energy for melting and refining glass is derived from Joule effect heating developed between electrodes extending through the bottom of the glass tank and distributed thereover in a suitable array. The discharge end of the tank is operated under continuous melting conditions of production with a limited amount of augmenting heat applied from the upper surface of the glass constituents such that the upper surface in the glass issuing or discharge region is molten and thus sufficiently fluid to permit and even induce the evolution of gas from the glass prior to its discharge. Glass batch is charged in a charge region spaced from the discharge region and by virtue of the following batch being charged, the molten glass flow, and the flow of gas toward the free upper surface of molten glass, the batch floating on the molten glass advances across that surface as a thermal insulating blanket over a major portion of the molten glass to provide essentially cold top melting in that major portion of the tank. Under stable operation, the interface between the batch blanket and the molten glass at the exposed upper surface of the glass mass within the tank stabilized so that melting and preliminary refining occurred as a cold top operation while further refining is enhanced over that previously realized with fossil fuel fired refining. The glass composition can therefore be tailored to take advantage of the retention of constituents by the barrier of the batch blanket, such as those introduced for fluxing, for a greater portion of the time the glass is in a molten state. Less gas is evolved in the refining since smaller quantities of the material releasing the gas are utilized in the batch. Dusting is reduced since heat from the region in which a molten glass upper surface is exposed causes the upper surface of the batch blanket to crust. Stack gas temperature is substantially reduced and the heat loss of the system greatly reduced. This improved type of operation, which produces an exposed, molten, upper surface on a portion of the glass, a batch blanket over a preponderance of the exposed upper surface of the constituents in the tank, and a radiant leading edge on the batch blanket at its interface with the exposed molten, upper surface, has been termed "radiant leading edge operation".
An object of the present invention is to improve the process of and apparatus for the melting of glass.
Another object is to enhance radiant leading edge operation.
A third object is to control the location of the radiant leading edge in a glass tank and thereby control the refining of glass.
A further object is to increase the efficiency of heat utilization in glass melting furnaces.
A further object is to reduce the gas evolution and dusting from glass melting operations while enhancing the quality of the glass.
A sixth object is to facilitate the modification of existing (hydrocarbon) fossil fuel burning furnace facilities toward electric glass furnace facilities while realizing advantages available from both forms of application of heat energy to continuous melting of glass.
A seventh object is to reduce or eliminate fossil fuel burners as heat sources in the discharge region of a glass melting furnace, particularly such a furnace operating in the radiant leading edge mode.